Description

This article explains how to use unicast RPF-check fail filters in a dynamic profile in contrast to statically configured fail filter.

Symptoms

If you have asynchronous routes that are failing due to unicast RPF check, you can use fail filters to bypass the failure.

The fail filter enables you to set criteria for packets you want to be passed in spite of failing the RPF check, such as DHCP packets, which are dropped by default.

Solution

Unicast RPF helps ensure that a traffic source is legitimate (authorized) by comparing the source address of each packet that arrives on an interface to the forwarding-table entry for its source address. If the router uses the same interface that the packet arrived on to reply to the packet's source, this verifies that the packet originated from an authorized source, and the router forwards the packet. If the router does not use the same interface that the packet arrived on to reply to the packet's source, the packet might have originated from an unauthorized source, and the router discards the packet, or passes it to a fail filter.

To configure a firewall fail filter:
***Items in <> are variable attributes which can be set according to network needs***

1. Create the filter.

[edit]
user@router# edit firewall family inet filter <filter_name>

2. Specify a term for the filter.

[edit firewall family inet filter <filter_name> ]
user@router# edit term <term_name>

3. Configure the match conditions for the filter.

[edit firewall family inet filter <filter_name> term <term_name> ]
user@router# set from <match_conditions>

4. Configure the actions to be taken for the matching packets.

[edit firewall family inet filter <filter_name> term <term_name> ]
user@router# set then <action>

5. (Optional) Repeat Steps 3 and 4 for additional filter terms.

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